The present invention relates to a position measuring device with a measurement representation and a scanning device for scanning the measurement representation.
Position measuring devices with a single-track absolute code, also called chain code or pseudo-random code, are increasingly being accepted. There, the measurement representation includes a code track with code elements of equal width arranged one behind the other in the measuring direction. The code elements are provided in accordance with a pseudo-random distribution, so that a defined number of sequential code elements respectively form a codeword, and all code elements a continuous sequence of different codewords. A new codeword, which unequivocally defines an absolute position, is already pending after a displacement path corresponding to the width of a code element. Such a position measuring device is extensively described in DE 39 42 625, for example.
It is disadvantageous here that there are relatively long areas with identical properties in connection with a relatively long measurement representation, i.e. with a large measuring area. If, for example, the position measuring device is a magnetic measuring device, the code elements include north and south poles. An area with many codewords of identical magnetization creates a continuous field, which makes evaluation more difficult and affects a measurement representation extending adjacent to it, in particular an incremental graduation, in an undesired manner.
It is pointed out in the literature by Stevenson J T M and Jordan J R: Absolute position measurement using optical detection of coded patterns, Journal of Physics E. Scientific Instruments, GB, IOP publishing, Bristol, vol. 21, No. 12, 1 Dec. 1988, pages 1140 to 1145, and Gxc3x6ze S: Sequentielle und zyklisch-sequentielle Codierung fxc3xcr Winkel -und Lxc3xa4ngeniessungen [Sequential and Cyclic-Sequential Coding for Angle and Linear Measurements], Technisches Messen [Technical Measuring], R. Oldenbourg, publ., Mxc3xcnchen, vol. 61, No. 9, Sep. 1, 1994, pages 343 to 345, that several partial codes can be arranged interlaced. Because of this it is possible to increase the distance between the scanning elements.
It was recognized in DE 43 09 881 C1 that it can be disadvantageous if several code elements with identical properties are arranged directly next to each other. For this reason the distribution of the code elements is inversely repeated. The position information is formed from the signal difference.
The position measuring device has the disadvantage that the actual position information is redundantly present in the code track and that therefore the opportunity of providing different codewords on a preset measuring length is reduced.
The position measuring device in accordance with JP 3-296620, in which following each codeword the inverse codeword with respect to it is applied in the measuring direction, has the same disadvantage.
It is known from EP 368 605 B1 to divide a piece of digital position information into two or more fragments of approximately the same length. To store this piece of position information on a graduation, the first bit of the first fragment, then the first bit of the second fragment, down to the first bit of the last fragment, are stored on the graduation. Subsequently, the second bit of the second fragment, then the second bit of the second fragment, down to the second bit of the last fragment, are stored on the graduation. This is repeated until the entire position information is stored. Because of this, the position information of the individual fragments is interleaved. But it is not yet assured by this that the fewest number of code elements is arranged in direct succession in the measuring direction.
It is therefore an object of the present invention to disclose a position measuring device in accordance with which it is intended to avoid a plurality of successive code elements with identical properties, while still being able to code a large meaement area absolutely.
This object is attained by a position measuring device that includes a scanning device and a measurement representation that is scanned by the scanning device. The measurement representation includes a code of a succession of code elements, which form at least a first codeword and a second codeword that define an absolute position, the first codeword and the second codeword are arranged interlaced with each other in that code elements associated with the second codeword are inserted into a succession of code elements associated with the first codeword. The code elements associated with the first codeword have inverse properties with respect to the code elements associated with the second code codeword.
The position measuring device in accordance with the present invention has the advantage that only a few adjoining code elements have identical properties.
The present invention will be explained in greater detail in what follows by the drawings, Lawn is in: